This invention relates generally to a cooking magnetron power supply system and, more particularly, to such a system using a full wave bridge inverter.
Most microwave ovens presently on the market use a 50 or 60 Hz LC power supply system along the lines described in U.S. Pat. No. 3,396,342 Feinberg issued on Aug. 6, 1968. This type of power supply, which is used in microwave cooking appliances from low power sub-compacts to combination electric range/microwave units, has existed for over twenty years.
Among the advantages of the Feinberg power supply system are the simplicity of using only four components and good control of the power factor. Disadvantages include the bulk (weight and size) need for controlling the power by the duty cycle only, non-continuous filament power at power levels other than 100%, high in-rush current and lamination noise. The bulk disadvantage of the Feinberg system results from the requirement for a 50 or 60 Hz transformer rating of about 1.2 KVA. Iron and copper weight of such a transformer typically weighs about 700 grams and occupies a volume of 1710 cubic centimeters. Additionally, a physically large capacitor is required as a necessary component when using such a transformer in order to provide constant current regulation of magnetron power against variations in line voltage.
A push-pull system has been used or proposed in connection with powering a cooking magnetron. Although the push-pull system avoids some of the disadvantages of the Feinberg power supply arrangement, such a push-pull system has included disadvantages such as high cost, complex logic, high voltage Darlington connected power transistors, reactive (i.e., power dissipative) snubber networks, inherent unbalance in volt second characteristics for each half cycle of operation being caused by uncontrolled turned-off characteristics of switching transistors, poor input power factor (for example, 0.6), high EMI generation, poor conversion, and higher cost magnetics. The higher cost magnetics corresponds to a design having a variable leakage transformer as a means of power control.
Another prior design is disclosed in U.S. Pat. No. 4,281,372 Kornrumpf issued on July 28, 1981. Although generally useful, that design included several disadvantages. In particular, the design was more complex and expensive than the Feinberg system. A peak current of over 4 amperes is required to permit the delivery of 700 watts of microwave power. This peak current tends to result in initial undesirable magnetron moding and eventual short term failure. Additionally, this design may cause undesirably high electromagnetic interference. This design also disadvantageously needed switching transistors which are required to handle large peak currents and hold off very high voltages. A further disadvantage of this design is that complex logic is required for controlling switch operation.
Accordingly, it is a principle object of the present invention to provide a microwave energy generating system having a power supply with low cost and low physical volume and weight.
A further object of the present invention is to provide a microwave energy signal system having a power circuit which has high efficiency, minimum EMI emissions and otherwise avoids or minimizes the problems of the power circuits discussed above.